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What is a therm?

If you must rely on fossil fuels to heat your home - whether directly by burning a gas or liquid (heating oil) in a furnace or indirectly through the mostly fossil-fueled electricity grid – then burning natural gas in your home furnace is the cleanest and most efficient way to do it. One of the most popular blogs on Mapawatt (and one of the fundamentals to understanding saving electricity) is “What is a kWh”. But the kWh is mainly used (at least in the U.S) to designate electricity usage. While you can represent the energy in natural gas with the units kWh, it is typically priced to customers and shown on the bill as the therm, short for 100,000 British Thermal Unit (BTU). A BTU is the amount of heat required to raise the temperature of a pound of water by one degree Fahrenheit. Also, 1 therm is equal to about 29.3 kWh.

On your natural gas bill, you probably pay by the therm. For instance, I pay $0.89 per therm. This value is determined by measuring the volume of gas that your natural gas meter records (in CCF, which stands for 100 cubic feet) by the BTU Factor. The BTU factor is a conversion factor that accounts for the fact that a volume of gas will change based on pressure. For instance, if you fill a balloon with natural gas in Florida (at sea level) and carry that balloon to Denver (the mile high city), then the volume of gas will have expanded because Denver is at a lower pressure than Florida. However, the heating content in the balloon is the same, because the amount of molecules (the mass) of natural gas haven't changed! The BTU factor takes into account this pressure change to determine the actual heating value in a volume of natural gas. But you don't really need to worry about this. All you need to care about for your bill is the amount of Therms listed.

The therms shown on your bill represent the heat content in the natural gas that flowed through your natural gas meter. The actual amount of heat energy that entered your living space from your furnace is a product of the heat energy in the natural gas (that is burned in the furnace) and your furnace’s efficiency. Obviously, the more efficient your furnace is, the more of the natural gas heat is transferred to the air that enters your home. A higher efficient furnace will transfer more of the heat given off my the combusion of natural gas into the air that flows from the furnace into your house. This is why it is important to have a highly efficient furnace!

The therm is the measure of a gas or liquid’s energy content which is given off in the form of heat when that gas or fluid is combusted. For pure substances like methane (basically natural gas is methane with a few other impurities), the energy content is known. Methane is composed of 1 Carbon atoms and 4 Hydrogen atoms, and when the methane molecule combusts in your furnace, it will give off a given amount of heat energy that is known by anyone with a ASHRAE handbook.

In my home, we don't just use natural gas to heat our home, we also use it to cook with on our stove and oven (heat our food) and in our hot water heater (heat our water). In my next home I plan on using less therms to heat my water because I plan on installing a solar thermal water heater. If you need any form of heat in your house, natural gas is the best fossil-fuel form. Don't worry too much about what a therm actually is, you just need to know how to find how many therms you use each month on your bill so you can work on lowering them!

I agree that standardization in the context of residential energy use would be very helpful, I personally would prefer the expression of energy for residential billing in gigajoules (1 GJ = 277.8 kW h = 9.480 Therms). Last month my energy usage was 26.05 GJ of natural gas and 3.09 GJ of electricity.
Perhaps we should refer to the gigajoule as a residential energy unit (REU) on the utility bills.
-RC

It's unfortunate that utilities report the energy content of natural gas in therms. If they used kWh instead, it would be much easier to compare it with electricity, the other major form of energy in the home. For example, if a therm is about 30 kWh, I know that's 50% more energy than my house consumes in electricity in a day (about 20 kWh). It also happens to be somewhat less energy than contained in a gallon of gasoline (about 35 kWh) or a gallon of heating oil (41 kWh). Using a common unit for energy content gives a much better feel for how much we're talking about.
David
www.energymetricsne.com
energy.shadypixel.com

I have three options for heat in the winter: electricity baseboard, a 60 gallon propane tank, and a traditional brick fireplace.
The house was built forty years ago by an electrician who favored baseboard; after all, in 1970 the first oil embargo was four years away, nuclear power was unstoppable, and the US was (and still is) the "Saudi Arabia of coal", with 25% of the world's proven reserves. An auxiliary propane heater was also installed in the house, plus the fireplace.
Fast forward to today, and I recently calculated that propane now costs just as much as electricity to heat the house (a gallon of propane costs $4.63/gallon, whereas the same amount of BTU energy from the baseboard costs about 17 cents per kWh, or "$4.70 per gallon" in propane terms). The electricity is also more efficient because the propane loses some heat when venting exhaust gases...so electricity costs less than propane in NJ (haven't done the efficiency calculations just yet for the propane heater).
However, I live in a wooded area, so I can purchase a cord of wood for $200. A seasoned cord of hardwood can have up to 28 million BTU's, as much energy as 300 gallons of propane ($1350). The only problem is maintaining the fire, which can't be done while away from the house (I also need to chop it, store it, and keep it dry). I know wood is not a resource that can be sustainable over the long run, but calculating the costs per BTU this past winter was amazing and enlightening.